Preheat the oven to 350°F. To make removing the cake easier, line a 9x9-inch square cake pan with parchment paper. Spray with a non-stick spray and set aside. In a medium sized mixing bowl, combine the flour, brown sugar, and cinnamon. Stir to combine and then add the melted butter. Blend the butter into the dry ingredients until it forms a crumble. Set aside while you prepare the remaining ingredients.
Caramelized Bananas
Peel the bananas and cut into a medium sized dice. Place a large non-stick skillet over medium heat. Melt the butter in the skillet and then add the brown sugar and heavy cream. Whisk until the mixture is fully emulsified. Add the salt and optional rum.
When the caramel comes to a boil, add the diced bananas. Cook the bananas for about 3 to 4 minutes, stirring occasionally, until they begin to soften. Remove from the heat before they start to break down. Use a slotted spoon to remove the bananas from the caramel sauce. There will be some caramel that clings to the bananas, which is fine, but try not to add too much of the sauce to the bananas. Reserve the remaining caramel sauce for later.
Caramelized Banana Coffee Cake
In a small mixing bowl, combine the flour, baking powder, baking soda, and salt. Whisk together and then set aside. In a large mixing bowl, combine the granulated and brown sugar. Add the eggs and melted butter and whisk until smooth. Add the vanilla extract and sour cream and whisk just until combined.
Add the dry ingredients to the wet ingredients. Stir just until the batter is combined, making sure not to over mix. Fold in the warm reserved bananas just until combined.
Pour half of the batter into the prepared baking pan and spread out evenly. Top with half of the crumble topping. Spoon the remaining batter over the crumble, gently spreading it out evenly. Top with the remaining crumble.
Bake the banana coffee cake for 40 to 45 minutes until a toothpick inserted in the middle comes out clean and the top springs back when touched. Transfer to a rack and let cool briefly before cutting. Serve the warm coffee cake with the reserved caramel sauce. Enjoy!